1. Field of the Invention
The present invention relates to electrical energy storage devices, such as electrochemical cells and capacitors. More particularly, the present invention relates to a casing member, such as a lid or cover, for the casing of an electrical energy storage device. Still more particularly, the invention relates to a casing member, such as a lid, for electrical energy storage device casings having at least one opening made by a fine blanking technique.
2. Description of Related Art
Electrochemical cells and capacitors typically include a container with an opening that is closed by a lid or cover to form a casing for the electrical energy storage device. Inside the container is an anode/cathode electrode assembly activated by an electrolyte. The container and the lid are of electrically conductive material and serve as a contact for either the anode or the cathode. In a case negative cell, the anode current collector is in contact with the casing while for a case positive design, the opposite is true. The other of the anode and the cathode not in contact with the casing is connected to a terminal lead or pin electrically insulated from the casing by a glass-to-metal seal. When a load is connected between the casing and the terminal pin of an electrochemical cell, a chemical reaction produces a voltage differential that generates an electrical current to power the load, for example, a medical device.
The lid must provide access to the interior of the casing for at least two purposes. First, the terminal pin connected to the anode current collector or the cathode current collector must pass through one of the lid openings to a position exterior of the casing. Second, the electrolyte must be filled into the housing through the other lid opening. Conventionally, two openings are defined in the lid for these purposes. The openings usually have structures connected to the lid to aid in sealing them. For example, a terminal lead ferrule is attached to the lid to accommodate the terminal pin and a fillport/closure assembly is used for sealing the fill opening.
FIG. 1 is a perspective view of a prior art electrochemical cell 10 including a terminal pin 12 sealed to the cell lid 14 by a glass-to-metal seal (GTMS) 16. FIG. 1A is a cross-sectional view taken along line 1A-1A of the ferrule 18, GTMS 16, and terminal pin 12 of the cell 10. The ferrule 18 is typically comprised of a cylindrical tube 20 that is welded to the lid 14 of the battery casing 22. The terminal pin 12 of the cell 10 (or a capacitor) passes through the center of the tube 20. A ceramic or glass is fused within the annular space formed by the inner wall of the tube 20 and the terminal pin 12, thereby forming the GTMS 16.
The depth 24 of the GTMS must be sufficiently long to withstand mechanical, thermal and environmental conditions that the cell 10 is subjected to over its life. In the prior art cell 10, the lid 14 is not thick enough to house the seal by itself. Hence, the use of a ferrule 18 extending past the bottom surface 26 of the lid 14 is required to reliably seal the terminal pin 12 in the GTMS 16.
Another example of a prior art electrochemical cell that includes a terminal ferrule extending below the bottom surface of the lid is described in U.S. Pat. No. 6,010,803 to Heller, Jr. et al. This lid is formed by a metal injection molding process which requires that the intersections between the terminal lead ferrule and the main body of the lid and between the fillport structure and the lid be slightly curved or “radiused.” Heller, Jr. et al. believes that radiused junctions facilitate the flow of material during the metal injection molding process. This eliminates areas of stress concentration that can cause the molded material to crack.
A casing member such as a lid formed by fine blanking according to the present invention has a higher density and, consequently, less porosity than a metal injection molded lid. Metal injection molded materials, as exemplified by Heller, Jr. et al.'s lid, require a binder. Even though technology advances have reduced the amount of binder required, metal density is still about 98% to about 98.5%, after curing. In contrast, a one piece lid according to the present invention made by fine blanking has a density of about 99.99%, and maintains acceptable mechanical properties required for glassing the terminal pin in the glass-to-metal seal. The comparatively decreased porosity of a metal injection molded lid can lead to fracturing during the glassing process and during use, which could lead to loss of hermiticity.
Another prior art lid or cover for an electrical energy storage device is described in U.S. Pat. No. 6,986,796 to Warchocki et al. This patent is assigned to the assignee of the present invention and incorporated herein by reference. Warchocki et al. describe a unitary device lid including a terminal ferrule and a fillport structure formed from a single blank of conductive material. The lid fabrication process of Warchocki et al. eliminates the need for welding a sleeve-shaped terminal pin ferrule and fillport to the lid. However, the lid of Warchocki et al. requires a relatively thick starting blank of metal, and then further requires extensive machining of the blank to provide the raised cylindrical structures for the terminal lead ferrule and the fillport structure. This machining process is time consuming, and thus expensive on a per-piece basis.
What is needed, therefore, is a casing member, such as a lid for an electrical energy storage device, that can be quickly fabricated with a minimum number of process steps, and that is optimal for subsequent formation of a terminal pin seal and a fillport seal. The lid must also be of a material having sufficient density to maintain hermeticity throughout the useful life of the electrical energy storage device.